This invention relates to functional fluid compositions having the ability to inhibit and control damage to mechanical members in contact with these fluid compositions.
A wide variety of functional fluids are known and utilized for many applications. Functional fluids have been used as electronic coolants, atomic reactor coolants, diffusion pump fluids, lubricants, damping fluids, bases for grease, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids and as filter mediums for air conditioning systems.
In many of the functional fluid compositions used for the above purposes there have been reports of damage to the fluid during use and to mechanical members, especially metallic members in contact with the fluid, as evidenced by a loss of weight of such members, due to the wearing away of metallic parts. Damage has been reported in aircraft hydraulic systems, gas turbine bearings, jet turbine control systems, steam turbine bearings, steam turbine control systems, electrohydraulic control systems and aerospace control equipment. Damage has also been observed on such materials as glass, Teflon, Mylar, Plexiglass and members constructed from other non-metallic materials.
In those instances where functional fluids are used in the hydraulic systems of aircraft and aerospace systems, such systems impose stringent requirements on the hydraulic fluid. Not only must these hydraulic fluids meet stringent use requirements but they must also satisfy FAA and other government requirements for fire resistance. Additionally, the hydraulic fluid must be capable of performing in the hydraulic system over an extended period of time without causing significant damage or functional impairment to the various conduits, valves, pumps, and the like, through which the fluid flows in the course of such use.
Damage caused by functional fluids contacting valves and other members has been attributed to the wearing away or erosion of the environment in contact with the functional fluid in a hydraulic system. Among the many undesirable results caused by such damage is a marked decrease in strength of the structural mechanical parts in the hydraulic system, such as pumps and valves, along with an alteration of the geometry of these parts. Such changes in the case of pumps can cause a decrease in pumping efficiency, and in the case of valves can cause faulty operations, excessive leakage and even hazardous conditions.
This damage necessitates costly and time consuming premature overhaul of mechanical parts. Additionally, metal removed from component metallic mechanical parts in contact with the functional fluid contaminates the fluids, causes filter clogging aind excessive filter replacement, and requires premature draining and replacement of the fluid in the system. The metal contamination can also cause a change in physical and chemical properties of the functional fluids.
Metal contaminants can also reduce the oxidative stability of a fluid, thereby adversely affecting fluid performance. In addition, metal contamination of the fluid can manifest itself in numerous other ways, including viscosity change, increased acid number, formation of precipitates, decrease in chemical stability and discoloration.
Another problem in the industry is the unavoidable contamination of aircraft and electrohydraulic control systems with chlorinated solvents used to clean the systems and components. A detailed discussion of this problem appears in Vickers 22nd Fluid Power Conference Report, Oct. 30, 1972, Section 4, Pages 25-29. Contamination by chlorinated solvents decreases the service life of functional fluids and accelerates damage, causing excessive internal leakage in hydraulic systems to a point of malfunction. No additive heretofore known has satisfactorily overcome the problems associated with chlorinated solvent contamination of functional fluids.
In the past, there have been reports of damage to valves and other metallic members which contact phosphate ester fluids. U.S. Pat. No. 2,470,792 proposes to overcome this damage problem by the inclusion of a small percentage of water in an aircraft hydraulic system. Unfortunately, while the presence of a small percentage of water reduces certain types of damage when incorporated in some phosphate ester hydraulic fluids, the presence of water can have a corrosive effect as well as an undesirable effect on the stability of the fluid.
U.S. Pat. No. 3,707,501 discloses the use of phosphonium compounds to inhibit erosion damage to the metallic environment containing hydraulic fluids. However, the lubricant compositions require relatively high concentrations of phosphonium compounds, which are very expensive. In addition, the phosphonium compounds may contribute to the destabilization of the functional fluid.
U.S. Pat. No. 3,679,587 discloses alkali salts of perfluorinated alkyl sulfonic acids as erosion inhibitors. However, since these compositions are ash containing materials, high temperature operation could lead to the formation of particulate matter in a hydraulic system.